Modern plants for the production of electronic components such as semiconductors, integrated circuits and the like utilize a variety of gases and gas mixtures of extremely high purity and corrosivity in the manufacturing processes. The gases are often conducted over relatively great distances in a manufacturing facility from storage cylinders and tanks through various processing rooms for carrying out various operations such as the many etching steps required in the processing of the electronic components. The piping network is a significant component of the manufacturing facility. To produce the piping network, a great many piping components are assembled, and this assembly is generally carried out by welding sections of pipes to each other, to pipe fittings, to valves, and to the other components of the piping network.
Because of the corrosivity of the gases to be conducted through the piping network, stainless steel is the most commonly used material for the piping, fittings, etc., and electropolished stainless steel is the preferred type of material. One concern associated with welding such materials is the likelihood of inducing impurities and corrosion sites at the weld sites due to improper control of the weld. The weld sites are known to be corrosion sensitive points, and therefore the weld sites must be very carefully controlled in order to provide a high quality weld. And because of the great number of welds in any given facility, consistency is a very important factor in any technique used to control the welding process.
Modern welding systems used for welding stainless steels have included tungsten inert gas welding, metal inert gas welding and laser welding, and for pipeline welding, a process known as orbital welding has become very common. Orbital welding is a variation of a tungsten inert gas welding system using an inert gas flowing through the pipeline and through the welding head. The welding electrode is housed in a welding head which encircles the pipe, and the electrode orbits around the pipe within the welding head. Such an apparatus is shown, for example, in U.S. Pat. Nos. 5,136,134 and 5,223,686 to Benway, et al.
Orbital welding systems also use an inert shielding and/or purging gas for protecting the weld site from oxidation during the welding process. For example, argon is a commonly used gas and is caused to flow through the pipeline during the welding process. The inert gas is also used to flow through the orbital weld head around the weld site on the exterior of the pipeline.
Controlling the flow of the inert gas in the welding process is a very important aspect of the control system used, and a great number of different problems can arise when the inert gas is not properly controlled. U.S. Pat. No. 5,396,039 to Chevrel et al discloses a system for welding pipelines using an orbital welding technique with high purity argon as the purging gas flowing through the pipeline. U.S. Pat. No. 5,425,492 to Thode discloses another orbital welding process using an inert gas to protect the weld site from oxidation. The patent suggests adjusting the flow rate of the gas flow in response to pressure changes near the weld. Still another such system is disclosed in U.S. Pat. No. 5,304,776 to Buerkel, et al. This system applies a continuously variable pressure and vacuum within the pipes of the inert gas flowing through the piping network according to the position of the orbiting welding electrode and the spacing between the tip of the welding electrode and the molten weld puddle, in order to compensate for gravitational effects on the molten metal during the welding process.
These prior patents have disclosed techniques for controlling the nature of the welds produced. Industrial facilities often specify standards for welds, and conditions needed in order to attain compliance with the standard. For example, standards such as ANSI B31.3 specify acceptable weld contours and finishes, and defects must be removed and rewelded and re-examined for compliance. Moreover, welds are often cut out randomly for inspection, and if the welds do not comply with the standards, significant lengths of piping may have to be removed and replaced.
The pressure and flowrate of the inert gas flowing in the piping network are two important variables which contribute greatly to the quality of the welds produced in orbital welding systems. For example, a given flow rate must be maintained in order to adequately protect the weld site against oxidation. The actual flow rate is a function of several factors, but primarily the size of the pipe being welded. In addition, the pressure of the inert gas must be maintained within narrow limits during the welding process. While the flow rate may be set at the desired rate and will remain essentially constant, control of the pressure is more difficult in such welding processes for several reasons. For example, with each weld, the pipeline is increasing in length and thus internal volume. In addition, different lengths of pipe sections also provide different internal volumes. Further, for different diameters of pipe, different volumes are encountered. Thus, in order to maintain a uniform sweeping of a weld site by the purge gas, the flow rate of the purge gas may range from about 5 cfh (cubic feet per hour) to almost 600 cfh, depending on the size of the pipe, but the gas pressure must stay within a rather narrow, low pressure range of about 1-5 inches of water.
Leakage from the piping network during the welding is also a significant variable which causes variation in the pressure of the purge gas. For example, during the welding some gas is allowed to leak at the butt joint, but as the weld progresses around the pipe, the joint gradually closes until the leak is stopped. This causes a gradual increase in the pressure.
In a typical welding setup, to help maintain a constant pressure in the piping network, a restrictor is used at the end of the piping network remote from the weld, and a sensitive pressure gauge is used to control the pressure. A commonly used pressure gauge is known as a Magnehelic gauge, and is connected to a T-fitting generally downstream of the weld for monitoring the pressure. Process operators monitor changes in the pressure to enable them to adjust the flow of the purge gas with changes in the monitored pressure. Unfortunately, changing of the flow does not occur quickly enough in many cases to compensate for changing pressures. As a weld progresses around the pipe, any fluctuation in the pressure can also cause a sag in the weld upon pressure reduction, or a blowout of the weld upon pressure increase.